1. Field of the Invention
The present invention generally relates to wireless communications. More particularly, the invention relates to handovers of user equipment (UE) under the control of a radio network controller (RNC) in a wireless communications system.
2. Description of the Related Art
In wireless communications systems, soft handovers are used to gradually transfer a user equipment (UE) connection from one base station to another base station without the user experiencing any interruption in communications. Hard handovers are used to, for example, switch between modes of operations or switch between base stations when a soft handover cannot be performed. In particular, inter-frequency hard handovers are used to hand over the connection between the UE and the radio access network from one frequency carrier or band to another. Although it may not always be apparent to the user, hard handovers typically involve some interruption in the connection between the UE and the base station(s).
The connection between the UE and the radio access network includes an uplink (UL) connection on a first carrier frequency and a downlink (DL) connection on a second carrier frequency different than the first carrier frequency. The UL connection may be with a first base station that is independent from a second base station used for the DL connection. (See WO 93/19537 for an example of such a system.) However, most wireless communication systems, including the Global System for Mobile Communications (GSM) and IS-95, are fixed duplex systems in which both of the UL and DL connections are with the same base station and use one of a plurality of predetermined duplex UL-DL pairs. Each 1:1 pair of UL and DL carriers has a constant, fixed offset between the UL and DL frequencies to prevent interference between base stations and between user equipment.
When the radio network controller wishes to perform an inter-frequency hard handover for a particular UE for any reason (such as interference, load, etc.), it identifies a new UL-DL duplex pair and the UL and DL frequencies for that particular UE are changed in tandem at the same time to the new UL–DL duplex pair. This handover procedure is performed and the entire duplex pair is changed even if it is only desired to change only one of the UL frequency or the DL frequency. It is not possible, for example, to keep the uplink connection uninterrupted and change only the downlink connection.
Prior art FIG. 1 illustrates the inter-frequency hard handover procedure performed in a network according to Release 4 of the specifications developed by the 3rd Generation Partnership Project (www.3gpp.org). See section 7.11 of 3GPP TS 25.931 v4.1.0 (2001–06) for a complete description of the hard handover. As shown by steps 7 and 10, the inter-frequency hard handover procedure includes a Physical Channel Reconfiguration procedure, which is itself an extensive procedure. See section 8.2.2 of 3GPP 25.331 v4.1.0 (2001–06). Prior art FIG. 2 illustrates the messages taken by the UE in the inter-frequency hardhandover. See also section 6.4 of 3GPP TS 25.303 v4.2.0 (2001–09). The inter-frequency hard handover according to Release 4 of the 3GPP specification always assumes a new allocation of the uplink carrier due to the pairing of the UL and DL carriers. Even when it is only necessary to change the carrier frequency for only one of the UL and DL connections, all of the steps shown in prior art FIGS. 1 and 2 (including the physical channel reconfiguration) will still be performed and the carrier frequency for the other connection will still be reallocated. The 3GPP handover procedure thus has the disadvantage that there will be unnecessary interruption while both of the carrier frequencies are re-allocated.